Klystrons are high power microwave amplifiers invented at Stanford University in the 1930s and used extensively in transmitters for Radar, UHF television, satellite communications, and as power sources for electron accelerators used in medicine and high energy physics. In a conventional klystron, a cylindrical electron beam, confined by an electromagnet, interacts with a number of resonant cavities, amplifying an input signal by 30-60 dB.
FIG. 1 is a schematic view of a conventional klystron. An electron gun 100 emits a beam of electrons that is accelerated by a gain cavity 102 and travels through a drift space 104 and an output cavity 106 before terminating in a collector 108. An output waveguide 110 is coupled to the output cavity 106. RF power from the electron beam propagates through the output waveguide 110 through an RF window 112 and to an RF load 114. In conventional klystron, the RF power has a fundamental frequency determined by design, and it is desired that this RF power at the fundamental frequency is maximized. For example, the klystron shown in FIG. 1 is the 5045 klystron used in the SLAC linac. It has a fundamental frequency of 2.856 GHz with a peak power of 65 MW and average power of 45 kW.